Does peripheral arterial disease always start in the legs?

Not always. Pad can technically refer to any arterial disease outside the aorta or coronary vasculature. It is frequently in the legs, but also commonly in the carotid arteries, and sometimes in the arteries of the arms or stomach organs. The last two are rarer. Pad is important because the underlying process that causes arterial blockages is essentially the same no matter which arteries are involved.
PDA. It is more common to occur in the legs than it is in the arm but you can also get narrowing and arterial disease that feeds the arteries to the arm. Disease in the heart which can cause heart attacks or in the brain which can cause strokes coming from the neck can also occur. We refer to peripheral artery disease as that affecting the legs and the arms. The legs are much more common.